Código: Seleccionar todo

package leer;

import java.util.Random;
import java.util.Scanner;

/**
 *
 * @author Orlando
 */
public class Leer {
    public static void main(String[] args) {
        System.out.println(".:.:.:.:.:.:.:.:.:.:.:.:..:.:.:.:.:.");
        System.out.println("        Generador de cadenas        ");
        System.out.println("            By xNeoDarkx            ");
        System.out.println(".:.:.:.:.:.:.:.:.:.:.:.:..:.:.:.:.:.");
        System.out.println("Introduzca la longitud de la cadena");
        System.out.println(RandomString(getInt()));
                // TODO code application logic here
    }
    
    private static String RandomString(int Longitud){
        int i;
        char letras[] = {'A','B','C','D','E','F','G','H','I','J', 'K','L','M','N','O','P','Q','R','S','T','U',
            'V','W','X','Y','Z','1','2','3','4','5','6','7','8','9','0','a','b','c','d','e','f','g','h','i','j',
            'k','l','m','n','o','p','q','r','s','t','u','v','w','x', 'y','z'};
        Random r = new Random();
        StringBuilder sTemp = new StringBuilder();
        
        for (i = 0; i < Longitud; i++){
            sTemp.append(letras[r.nextInt(letras.length)]);
        }
        return sTemp.toString();
    }
    
    private static int getInt(){
        Scanner ReadLn = new Scanner(System.in);
        return Integer.parseInt(ReadLn.next());
    }
}